Implantable medical device with low power delta-sigma analog-to-digital converter

Abstract

In general, this disclosure describes techniques for reducing power consumption within an implantable medical device (IMD). An IMD implanted within a patient may have finite power resources that are intended to last several years. To promote device longevity, sensing and therapy circuits of the IMD are designed to incorporate an analog-to-digital converter (ADC) that provides relatively high resolution output at a relatively low operation frequency, and does so with relatively low power consumption. An ADC designed in accordance with the techniques described herein utilizes a quantizer that has a lower resolution than a digital-to-analog converter (DAC) used for negative feedback. Such a configuration provides the benefits of higher resolution DAC feedback without having the use high oversampling ratios that result in high power consumption. Also, the techniques avoid the use of, and the associated high power consumption of, a high resolution flash ADC, within the sigma delta loop.

1. An implantable medical device comprising:
 at least one sensor that generates an analog input signal; 
 at least one delta-sigma analog-to-digital converter (ADC) that converts the analog input signal to a digital signal, the delta-sigma ADC comprising:
 an integrator that produces an integrated difference between the analog input signal and a reconstruction of the analog input signal; 
 a quantizer that produces a control signal based on the integrated difference; 
 an up/down counter that adjusts a digital value according to the control signal; 
 a digital-to-analog converter (DAC) that generates the reconstruction of the analog input signal based on the digital value of the up/down counter and provides the reconstruction of the analog input signal as negative feedback to the integrator, wherein a resolution of the quantizer is less than a resolution of the DAC; and 
 a compensation module that compensates for a phase shift in the negative feedback caused by the integrator and the up/down counter, 
 
 
     wherein the device further comprises:
 a processor that receives the digital signal from the delta-sigma ADC. 
 
   
   
     2. The device of  claim 1 , wherein the resolution of the DAC includes at least four times as many bits as the resolution of the quantizer. 
   
   
     3. The device of  claim 1 , wherein the quantizer comprises a comparator having a 1-bit resolution. 
   
   
     4. The device of  claim 3 , wherein the DAC has at least a 6-bit resolution. 
   
   
     5. The device of  claim 1 , wherein the compensation module introduces one of a zero and a pole-zero pair into a transfer function of a feedback loop formed by the DAC in one of an analog domain and a digital domain. 
   
   
     6. The device of  claim 1 , wherein the integrator comprises a continuous time Gm/C differential integrator that integrates the difference between the analog input signal and the reconstruction of the analog input signal and provides the integrated difference to the quantizer, wherein the quantizer comprises a differential input quantizer. 
   
   
     7. The device of  claim 1 , wherein the processor controls delivery of a therapy to a patient based on the digital signal from the delta-sigma ADC. 
   
   
     8. The device of  claim 7 , wherein the processor determines whether an intrinsic depolarization occurs within a time interval based on the digital signal from the delta-sigma ADC and delivers the therapy to the patient based on the determination. 
   
   
     9. The device of  claim 1 , wherein the sensor generates an analog input signal representative of a sensed physiological parameter of a patient. 
   
   
     10. The device of  claim 1 , wherein the implantable medical device includes one of a cardiac pacemaker, a cardiac defibrillator, an electrical neurostimulator, an implantable drug delivery device, and a monitoring device. 
   
   
     11. A method comprising:
 receiving an analog input signal from at least one sensor of an implantable medical device; 
 converting the analog input signal to a digital signal using at least one delta-sigma analog-to-digital converter (ADC), wherein converting the analog input signal to the digital signal using the at least one delta-sigma ADC comprises:
 producing, with an integrator, an integrated difference between the analog input signal and a reconstruction of the analog input signal, 
 producing, with a quantizer, a control signal based on the integrated difference, 
 adjusting a digital value of an up/down counter according to the control signal, 
 generating the reconstruction of the analog input signal based the digital value of the up/down counter using a digital-to-analog converter (DAC), wherein a resolution of the quantizer is less than a resolution of the DAC, 
 providing the reconstruction of the analog input signal as negative feedback to the integrator, and 
 compensating for a phase shift in the negative feedback caused by the integrator and the up/down counter, 
 
 
     wherein the method further comprises:
 sending the digital signal from the delta-sigma ADC to a processor for analysis. 
 
   
   
     12. The method of  claim 11 , wherein the resolution of the DAC includes at least four times as many bits the resolution of the quantizer. 
   
   
     13. The method of  claim 11 , wherein the quantizer comprises a comparator having a 1-bit resolution. 
   
   
     14. The method of  claim 13 , wherein the DAC has at least a 6-bit resolution. 
   
   
     15. The method of  claim 11 , wherein compensating for the phase shift comprises introducing at least one of a zero and a pole-zero pair into a transfer function of a feedback loop formed by the DAC in one of an analog domain and a digital domain. 
   
   
     16. The method of  claim 11 , wherein producing, with the integrator, the integrated difference comprises:
 integrating the difference between the analog input signal and the reconstruction of the analog input signal using a continuous time Gm/C differential integrator; and 
 providing the integrated difference to a differential input of the quantizer. 
 
   
   
     17. The method of  claim 11 , further comprising controlling delivery of a therapy to a patient based on analysis of the digital signal from the delta-sigma ADC. 
   
   
     18. The method of  claim 17 , wherein controlling delivery of a therapy to a patient based on analysis of the digital signal comprises:
 determining whether an intrinsic depolarization occurs within a time interval based on the digital signal from the delta-sigma ADC; and 
 delivering the therapy to the patient based on the determination. 
 
   
   
     19. The method of  claim 11 , wherein receiving an analog input signal from at least one sensor comprises receiving an analog input signal representative of a sensed physiological parameter of a patient. 
   
   
     20. The method of  claim 11 , wherein the implantable medical device includes one of a cardiac pacemaker, a cardiac defibrillator, an electrical neurostimulator, an implantable drug delivery device, and a monitoring device. 
   
   
     21. An implantable medical device comprising:
 means for generating an analog input signal; 
 means for converting the analog input signal to a digital signal, wherein the converting means comprises:
 means for producing an integrated difference between the analog input signal and a reconstruction of the analog input signal, 
 means for producing a control signal at a first resolution based on the integrated difference, 
 means for adjusting a digital value according to the control signal, 
 means for generating the reconstruction of the analog input signal at a second resolution based on the digital value and providing the reconstruction of the analog input signal as negative feedback to the means for producing the integrated difference, wherein the first resolution is less than the second resolution, and 
 means for compensating for a phase shift in the negative feedback caused by the means for producing the integrated difference and the means for adjusting the digital value, 
 
 
     wherein the device further comprises:
 means for analyzing the digital signal from the converting means. 
 
   
   
     22. The device of  claim 21 , wherein the second resolution includes at least four times as many bits as the first resolution. 
   
   
     23. The device of  claim 22 , wherein the first resolution is a 1-bit resolution and the second resolution is at least a 6-bit resolution. 
   
   
     24. The device of  claim 21 , wherein the means for compensating comprises means for introducing one of a digital zero and a pole-zero pair into a transfer function of a feedback loop formed by the generating means in one of an analog domain and a digital domain. 
   
   
     25. The device of  claim 21 , wherein the means for producing an integrated difference comprises means for integrating the difference between the analog input signal and the reconstruction of the analog input signal in continuous time, wherein the integrating means outputs an integrated differential signal to a differential input of the quantizer.